1. Field of the Invention
This invention relates to a process for coating heat resistant substrates with aqueous stoving compositions based on organic solutions or dispersions of polyacrylate resins and organic polyisocyanates and to the coated substrates produced therefrom.
2. Description of the Prior Art
Reduction of the organic solvent content of lacquers is a primary concern in many fields of application for ecological and factory hygiene reasons. Accordingly, the at least partial replacement of organic lacquer solvents by water is of considerable importance in modern coating technology.
The stoving compositions predominantly used for industrially applied coatings, for example, for coating metals (original and repair lacquering of automobiles, coating of domestic appliances, etc.) either require stoving temperatures of 130.degree. C. and higher (for example melamine resin crosslinking, powder coatings, epoxy resin crosslinking) or, as non-aqueous systems, contain relatively large quantities of organic solvents (for example two-component polyurethane lacquers). Accordingly, there is a considerable need for lacquer systems which can be stoved at relatively low temperatures, i.e., energy-saving lacquer systems, and which can be quickly and completely crosslinked to hard, elastic and solvent-resistant coatings.
Accordingly, an object of the present invention is to provide a new coating composition which can be applied from a predominantly aqueous phase and which can be cured at only moderately elevated temperatures to obtain high quality lacquer films without the release of decomposition products, for example, blocking agents for polyisocyanates.
Lacquer systems of this type have not previously been known. For example, the water-based lacquer systems according to DE-OS 2 507 842 or DE-OS 2 507 884 rely on the use of blocked polyisocyanates while the systems according to DE-OS 38 29 587 are cured at room temperature without any indication as to how the systems would have to be modified to obtain high-quality stoving lacquers. The same observation is justified in regard to the paper entitled "WaBrige Zweikomponenten-Polyurethan-Reaktiv-Systeme in der Beschichtungstechnik (Water-Based Two-Component Reactive Polyurethane Systems in Coating Technology)"(proceedings of the XX FATLPEC Congress, Nizza, 17th to 21st September, 1990, pages 239 to 245).
The object stated above may be achieved in accordance with the present invention which is described in detail hereinafter.